The present invention concerns a process for the manufacture of organic isocyanates from nitro compounds and, more particularly, a process for the preparation of aromatic isocyanates by the reaction between aromatic nitro compounds and carbon monoxide.
Aromatic isocyanates are organic intermediates of great interest. Two among them are particularly well known, namely, toluene diisocyanate and diphenylmethane-4,4'-diisocyanate which are used in the synthesis of polyurethanes. The industrial processes for the preparation of these products all involve the phosgenation reaction of an amine coming from the catalytic hydrogenation of a nitro derivative. The disadvantages of these processes are many. Specifically, they require the synthesis and manipulation of phosgene, a very dangerous product, and they produce hydrochloric acid in large quantities and therefore require the erection and costly maintenance of a special installation for the electrolysis of this acid in order to recycle the chlorine.
The advantage presented by a process avoiding the use of phosgene is obvious. Catalytic compositions permitting the preparation of isocyanates by the reaction of an organic nitro compound with carbon monoxide, at high temperature and pressure, are described in several patents. It is thus that the use, as catalyst, of a noble halide in the presence of a basic aromatic amine is described in French Pat. No. 1,600,529. Catalysts formed of halides or oxides of Ru, Rh, Pd, Os, Ir, Pt and of a heteroaromatic sulfur-containing compound in the presence possibly of an oxide of Cr, Mo, Nb, W, and V are described in West German Pat. No. 1,910,303. The use of a catalytic system formed of a noble metal halide and of an organic phosphorous compound, for instance a triaryl phosphine or a phosphite, is described in French Pat. No. 1,567,321. Catalytic systems composed of one or several palladium and/or rhodium halides, one or several heteroatomic nitrogenous bases and of a co-catalyst formed of one or several iron borates are described in French Pat. No. 2,155,242. In French Pat. No. 2,120,110 besides a palladium halide and heterocyclic nitrogenous bases, the catalytic formulation includes a co-catalyst composed of one or several iron and/or manganese molybdates. Certain attempts have been made to deposit the active phases on supports; it is thus that French Pat. No. 1,600,529 and No. 1,558,896, as well as U.S. Pat. No. 3,728,370 envision, without practical illustration, the deposition of the catalyst on supports of the type of alumina, silica, carbon, barium sulfate, calcium carbonate, asbestos, bentonite, diatomaceous earth, and Fuller's earth. The use of palladium, palladium chloride or rhodium chloride on alumina, silica and silicon carbide for the carbonylation, in the vapor phase, of nitro derivatives is described in British Pat. No. 1,257,932, and a publication by W. B. Hardy and R. P. Bennett in Tetrahedron Letters No. 11, p. 961 (1967) gives the results obtained with a catalyst consisting of rhodium on carbon, to which FeCl.sub.3 had been added, in the carbonylation of nitrobenzene to form phenyl isocyanate.
All of these systems based on precious metals make it possible to produce isocyanates from nitro compounds with variable selectivities and productivities. However, their difficult and costly recovery prevents commercial industrial use.